The Undo event for controls occurs whenever the user returns a control to its original state by clicking the Undo Field/Record button on the command bar, clicking the Undo button, pressing the ESC key, or calling the Undo method of the specified control. The control needs to have focus in all three cases. The event does not occur if the user clicks the Undo Typing button on the command bar.
Private Sub Form_Undo(Cancel As Integer)
Dim intResponse As Integer
Dim strPrompt As String
strPrompt = "Cancel the undo operation?"
intResponse = MsgBox(strPrompt, vbYesNo)
If intResponse = vbYes Then
Cancel = True
Cancel = False